With the gradual development of science and technol ogy, computer systems become more and more complica ted. Meanwhile, computer system security is not well gua ranteed. A number of lawbreakers take advantage of computer system vulnerability, which is a threat to computer system security. Computer system security cannot b e strengthened unless assessments and researches on c omputer system vulnerability are carried out. This paper aims to cover the definition of computer system vulnerab ility and explain the causes of the problem. At las t, this paper will conclude several methods of doing computer sys tem vulnerability assessment.